Ed Anderson
Joined: 10 Jan 2006
Posts: 475
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 Posted: Mon Dec 24, 2007 9:47 am Post subject: Braking question was Brakeline Failure Photos |
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Hi Steve,
Reasonable questions. At the time, I had over 50 hours on those brake pads
and no indication of dragging.
No, I had started my take off roll and had almost reached lift off when the
engine started to miss. Since I was on a 4000 foot runway, I decided to
abort the take off to check things out before setting off on my 800 mile
return trip to NC. So I aborted and coasted down to the far end which was
on a downhill grade and everything seemed normal as I braked for the turn
around. As I pushed on the right brake peddle to turn I noticed the brake
seemed mushy. On the way back, I gunned the engine to check out the engine
miss and was probably riding the brakes a bit to keep the speed down, when
the right brake peddle goes to the floor board. Things really got exciting
after that point. Loss of steering, airplane on fire and off runway
excursion!!
I did not think at the time I had ridden the brakes that hard and I may or
may not have. But when I did a bit of research, I found the flash point of
the brake fluid was between 215F and 240F - believe me. any braking at all
can get your disc up to that temperature easily. I also posted to the list
the same thing happening on an RV-8A. The low flash point of the brake
fluid is the reason I then switched to the newer brake fluid with a flash
point of nearly 440F. So did my braking contribute to the temps of the
disc - no question it did, was it unusually braking - I didn't think so at
the time or now - but its clear that any disc temp over 240F could have
cooked off the fluid. But, it would have been a moot point, had the brake
line not broke and sprayed fluid on the hot disc.
I did a quick calculation of the kinetic energy dissipation required to
bring my 1700 lb aircraft to a stop from 60 mph. If I did the calculations
correctly it indicated I could get a disc temp max temperature increase of
330F. The temp depends of course, on how hard I braked or how long the slow
down took. So that is probably the maximum increase, but even if it were
1/2 that rise in temp, if starting out with a ambient temp of 80 + 330/2 =
240F which is above the fluid's flash point. So it does not take a lot of
braking to get those disc temps up.
That is also one reason I discarded the old discs and went with ones that
were twice as thick as the standard van discs. Having more disc material to
absorb the same amount of heat will keep the disc temp lower.
YMMV
Ed
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_________________
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com |
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